ganuganu wrote:need to measure pulsejet engine thrust. I made several thrust tests horizontal and vertical but iam not satisfied with it, so i want to make a stand which is well and accurate to certain point.

Okeyy ... First, the reason why I said "OMG ... Here come that question again ... Bla bla bla" is because everytime there is discussion on test stand here, it usually end-up in a "Debate", often leading nowhere and where peoples argue over their own concept and comprehension ... So maybe just expect someone to jump in all arms up saying "No no no ... that not true" ... End of briefing on the matter.

This said, I will now present you a few "Facts" from which I am convince, you will have all the knowledge and common sense to understand.

Fact #1
Gas turbines delivers a steady thrust, like you would lean on your car to push it with a constant and steady pressure. In opposite, pulsejets deliver "Pulsed" thrust at frequancies, a bit like driving a pile in the ground using a sledge hamer (Bang, Bang, Bang ... Blows or Pulsations).

The instant you understand your pulsejet delivers a momentary thrust or blow, then relaxes (No more pressure), then ingest air and fuel mixture from BOTH its intake and tail pipe (Negative thrust) to then go "Bang" and push again, you have all the comprehension to logically understand and figure out the following.

This understood, your loadcell choice becomes crucial. "Strain" type loadcells are made to measure steady pressure and therefore, not a good choice to measure pulsejet thrust. In opposite, "Piazo" or "Dynamic" or "Quartz" loadcells measure "Impacts" or "Blow" and therefore, are the best to measure a pulsjet thrust.

Most of the strain loadcell read and measure apprx. 3 to 6 Hz, some more sophisticated can be faster, but none as fast as a 200, 300 or 400 Hz runing pulsejet. Therefore, trying to read "Pulsed" thrust should bring you to use a dynamic loadcell. Some here will tell you "Heyyy ... I know a strain loadcell that does", but ask him to give you the price and he will have made my point.

Fact #3
All material on this planet have an "Elastic Modulus" or "Flex Modulus", meaning that even if you take a 2" thick steel plate and give it a hamer blow ... IT WILL FLEX ... no matter what. Everything first Flex (Elastic Modulus and Yield Strenght) and then break if their strength is exceeded ... That's a Fact. This is so true, that some PDE engine research I'm aware off, measure their engine's thrust ONLY from the first "Bang", because they know all the other pulse measurments are not accurate because the test bed enters into "Momentum" oscilation screwing up the readings, no matter what size or strength the test bed has.

Action / Reaction; Is another hurtle with measuring pulsejet thrust. Take a basketball hung by a rope to the roof. Now, give that ball a push, and what happen? The ball move away from you then comes back and hits you ... Right?. Now, hold a loadcell between your hand and the ball and give the ball another push, and what happen? As you push the ball you're reading a pressure from the loadcell and the ball again moves away. BUT ... BUT ... BUT ...As you are doing NOTHING (No pressure indicated) the ball comes back, hits the loadcell in your hand and here you are reading another pressure ... WHILE YOU DID NOTHING. ... That's the main core problem with Pulsejets, test stand and thrust measurements ... Period ... Nothing else to add ... And that a HELL of a problem.

The same happens to our above friends with their PDE engine. As their engine goes "Bang" and pushes, the test bed flex in the same direction (Reading X thrust = True). As the engine relax, the test bed relax too and comes back to its original position BUT after and later then the engine (Delay between engine and bed momentum). As the engine goes "Bang" again, it goes AGAINST the bed still coming back to its "Relax" position and cause from the delay, thus, the thrust reading 2X thrust (False) ... And the bed and engine goes into oscilation or runing after each other, and so on, and so on ... BUT NEVER ACCURATE.

The first clue I had of this, was years agao from a first test bed partially mounted on inflatable wheels to move it around. Taking engine #1, I read X thrust, but also noted the stand was oscilating back and fort, an exagerated motion due to the rubber wheel. Luck me this stand was on wheels, otherwise I would have probably never noticed the problem. I then mounted the stand on concrete block and the same engine at same power demonstrated different thrust readings. I then went to a pendulum concept and the thrust was again different. I then went to a solid and rigid test bed, and heretoo, got different thrust readings, always with the same engine, although more accurate. I then decided I had waisted enough time on this, decided to talked to experts, spent the money and stop messing around, and get a real working DAS program done by experts, and FINALLY ... Got "Some" decent accuracy at last. In all, we probably spent I don't know, maybe between 10K to 15K on that stand and program, and still, we knew we still had a certain degree of innacuracy.

SO ... Here you are my friend ... You just realized and learned ; 1) Why it is SO HARD to adequatly measure thrust from a pulsejets, 2) That 99.99% of the Backyard made thrust stand are innacurate, 3) That any suggestions making a stand using cables, oil dampers, counter balance or counter weight, a plain fish scale or a pendulum concept (Remember our ball), are TOTALY innacurate, just providing you with a "Slite" and very slite idea of what your engine is delivering.

It always amazes me when I read "Heyyyy guys ... My pulsejet delivers XX pound of thrust" and I see their stand ... I then go "Yeaaaaa ... Shureeee".

The solution
I will not pretend having the perfect solution and maybe all the above was just to tell you what "NOT TO DO" nor waist your money on. But I can tell you from having spent alot of time and money addressing the issue, including extensive consultation with our National Propulsion Research Department and DAS experts, that your test stand should be somewhere like this :

- Using a "Dynamic" Loadcell (Forget cables, oil dampers, counter weights, fish scale or pendulum).
- Be as heavy and strong as possible (Flimsy steel makes test beds and innacurate readings due to oscilation).
- Equipped with a computer in which to connect and read the "Dynamic" loadcell.
- This computer should have a little DAS (Data Acquisition System) program. If you're still going on and decide to go this way, I still have the DAS program, this program including filters to rule out pulse engine's "Dead" and "Negative thrust" cycles, but you will need to use the "Softwire" program.

Finally, remember this one ; The best and "Perfect" thrust measurement is only the "First" impulse, the rest just being average indication from an ongoing CAOS ... Your engine ...

OR ... Another way I would think of (Never did it myself), is to have your engine mounted on an airframe, record the total mass, have the plane flying perfectly leveled at max. power while a cop (If you know a friendly one) records the speed using his radar (Be usefull for once ... Joke ), so you can mathematically calculate the "Force" having factors like "Velocity", "Mass", current atmospheric condition and total airfram/engine resistance. Maybe a way to do it cheaper.

In fact, pulsejets are so hard to measure thrust wise, that for all the cases, accurate thrust reading is directly linked to your wallet. More you have to spend, more accurate your readings will be ...

So here is my other question : How accurate (%) you want your stand to be and how much do you want to pay for that accuracy?

I hope the above is appropriate and complete, best of luck and feel free to writte if you need more information,

The accelerometer act in providing the computer with the engine's operating frequency for "Trigering" purposes, so the computer knows when to read the dynamic loadcell and when not too.

Some will ask; Why not use the loadcell signal for triger too? And the answer is; Having the loadcell looped into providing thrust reading and trigger delays the computer's process, and here, computer sped is crucial for this purpose.

Also and as found, you will need a hell of a computer, something like a high process rate dual or quadrupal core CPU.

PyroJoe wrote:Not a debate question, but what fraction of engine mass should the test stand mass not exceed?

This is not a debate question at all. In fact, it's quite pertinent ... And it could be addressed both direction (i.e. : Test stand to engine or engine to test stand).

This said, your question is quite right and a very important one. To often we see a 5 pound heavy engine delivering 20 lbs. of thrust, mounted on a 50 pound test stand. I even saw a guy tie is stand with wires that after a few minute, broke and the engine was all over the place and almost injured the guy.

Not having the accurate answer or maths to your question, although I fully understand and acknowledge it, I can tell you that my last test bed was close to 1000 lbs. and it took a lift truck to bring it in, for testing a 40 pound heavy engine ... But that's one factor only.

The second aspect to consider is "Expected thrust". If the bed is a 1000 lbs. heavy, the engine is 40 lbs., but it is expected the engine to deliver some where around 350 lbs. of "Pulsated" thrust, then, you end-up in the same problem.

Then, there's the "Engine mount"; You can have a very rigid bed, but the engine is coupled to the bed using a flimzy mount and your problem as moved to the mount. The loadcell becomes the problem too at some point. Although very rigid, loadcells also flex, otherwise they would not work, and your problem moves again.

One of the best test bed (Most accurate) I saw, was not only VERY massive, but the engine was held in place by a massive "Pillow Clamp" type mount covering the entire chamber and was over 12" long, very massive too. And instead of using loadcells, they were using accelerometers to measure "Displacements". As they knew all material characteristics (Yield and tensile Modulus), they were able to calculate the amount of force required to create the "Displacement" and extrapolate thrust figures. Personally, I think this is the best way of doing it. Jam every thing in a solid and massive block, and measure displacement (Motion) through the material.

Like I said above, I do not pretend knowing the exact solution or formulas. My goal was just to tell the guy what he was getting into and provide hime with choices to have the best (What is the best?) results, highliting the fact that "Pulse-jets" (Of any types) are the hardest engines to measure thrust.

But again, my first goal was just to "Help", for I left "PJs" behind for a while now. Having spent 10 years messing with these, I figured I had enough and carried on with other stuff. I'm just sharing what my last path allowed me to learn, and you will not see me spend a single minute with these anymore, other then help if requested.

With the low tech fish spring scales I typically like to see the engine and cradle to be rigid, but very lightweight, and the base to be rock solid. Knowing the measure isn't significantly accurate, (and typically my engines are very heavy) I rarely will mention thrust.

Placing the engine on a light weight vehicle and going through the F=m*a set may be the cheapest path.

PyroJoe wrote:Placing the engine on a light weight vehicle and going through the F=m*a set may be the cheapest path.

Totally true and you can't say better then this I guess. If you start considering all the Pro,s & Con's when trying to measure pulsejets thrust, you start understanding how difficult these engines are to measure.

Personnally and if I had to play with these again, I would go for the MASSIVE bed with accelerometers instead of loadcells, not that your point is not the best, but here in Canada, things get very complicated when talking about flying these engines, the noise, the speed they can produce .... All sorts of assles.